The requirements of satellite and space probe borne super cooled instrumentation for cryogenic cooling fluids necessitate provision for containing the necessary cryogen supply during launch and for periods up to a year or more, thereafter. An example of such an instrument is an infrared sensor which requires cryogenic liquid cooling to obtain optimum infrared radiation sensitivity. Such on-orbit instrumentation must be maintained at temperatures of typically 10.degree.-140.degree. K. The use of cryogens avoids the power usage and complexity of a powered cooling system.
Prior art orbital cryogenic systems required maintenance of the cryogen in the frozen state. If the cryogen is allowed to liquify, the vent port of the cooler can become blocked with liquid, resulting in the liquid being immediately pumped out to space, depleting of cryogen and introducing safety hazards at the vent exhaust. This condition can readily occur due to vibration during launch and weightlessness during orbit. Freezing of the cryogen requires cooling coils, coolant supply, and regulation equipment and instrumentation to assure operators that the cryogen is maintained in a frozen state, adding weight and complexity to the spacecraft. Since the frozen cryogen must be kept at a vapor pressuer below its triple point, generally below one atmosphere of pressure, a pumping system must be incorporated if the solid cryogen is to be maintained on the launch pad beyond the limited amount of time before heat leak of the system results in cryogen melting. These pumping systems are heavy, require power, add complexity to the system design and operation, decrease system reliability, and create safety problems. Without such pumping systems, the launch vehicle carrying the cryogenic device can remain on the launch pad for only a limited amount of time without servicing of the frozen cryogen dewar. Procedures to freeze and subcool the cryogen also add complexity and time to launch pad operations, which are normally time and safety critical. Another drawback of the frozen cryogen system is that its cryogen cannot be replenished in orbit or in space by service vehicles, a requirement if sensors are to remain useful beyond a relatively short time in space. The above-mentioned disadvantages and limitations of the prior art frozen cryogenic system could be overcome if a practical liquid cryogen system could be provided.